HotSpot Coding Style

## Introduction

This is a collection of rules, guidelines, and suggestions for writing HotSpot code. Following these will help new code fit in with existing HotSpot code, making it easier to read and maintain. Failure to follow these guidelines may lead to discussion during code reviews, if not outright rejection of a change.

### Changing this Document

Proposed changes should be discussed on the HotSpot Developers mailing list. Changes are likely to be cautious and incremental, since HotSpot coders have been using these guidelines for years.

Substantive changes are approved by rough consensus of the HotSpot Group Members. The Group Lead determines whether consensus has been reached.

Editorial changes (changes that only affect the description of HotSpot style, not its substance) do not require the full consensus gathering process. The normal HotSpot pull request process may be used for editorial changes, with the additional requirement that the requisite reviewers are also HotSpot Group Members.

### Why Care About Style?

Some programmers seem to have lexers and even C preprocessors installed directly behind their eyeballs. The rest of us require code that is not only functionally correct but also easy to read. More than that, since there is no one style for easy-to-read code, and since a mashup of many styles is just as confusing as no style at all, it is important for coders to be conscious of the many implicit stylistic choices that historically have gone into the HotSpot code base.

Some of these guidelines are driven by the cross-platform requirements for HotSpot. Shared code must work on a variety of platforms, and may encounter deficiencies in some. Using platform conditionalization in shared code is usually avoided, while shared code is strongly preferred to multiple platform-dependent implementations, so some language features may be recommended against.

Some of the guidelines here are relatively arbitrary choices among equally plausible alternatives. The purpose of stating and enforcing these rules is largely to provide a consistent look to the code. That consistency makes the code more readable by avoiding non-functional distractions from the interesting functionality.

When changing pre-existing code, it is reasonable to adjust it to match these conventions. Exception: If the pre-existing code clearly conforms locally to its own peculiar conventions, it is not worth reformatting the whole thing. Also consider separating changes that make extensive stylistic updates from those which make functional changes.

### Counterexamples

Many of the guidelines mentioned here have (sometimes widespread) counterexamples in the HotSpot code base. Finding a counterexample is not sufficient justification for new code to follow the counterexample as a precedent, since readers of your code will rightfully expect your code to follow the greater bulk of precedents documented here.

Occasionally a guideline mentioned here may be just out of synch with the actual HotSpot code base. If you find that a guideline is consistently contradicted by a large number of counterexamples, please bring it up for discussion and possible change. The architectural rule, of course, is "When in Rome do as the Romans". Sometimes in the suburbs of Rome the rules are a little different; these differences can be pointed out here.

## Structure and Formatting

### Factoring and Class Design

- Group related code together, so readers can concentrate on one section of one file.

- Classes are the primary code structuring mechanism. Place related functionality in a class, or a set of related classes. Use of either namespaces or public non-member functions is rare in HotSpot code. Static non-member functions are not uncommon.

- If a class `FooBar` is going to be used in more than one place, put it a file named fooBar.hpp and fooBar.cpp. If the class is a sidekick to a more important class `BazBat`, it can go in bazBat.hpp.

- Put a member function `FooBar::bang` into the same file that defined `FooBar`, or its associated .inline.hpp or .cpp file.

- Use public accessor functions for member variables accessed outside the class.

- Assign names to constant literals and use the names instead.

- Keep functions small, a screenful at most. Split out chunks of logic into file-local classes or static functions if needed.

- Factor away nonessential complexity into local inline helper functions and helper classes.

- Think clearly about internal invariants that apply to each class, and document them in the form of asserts within member functions.

- Make simple, self-evident contracts for member functions. If you cannot communicate a simple contract, redesign the class.

- Implement classes as if expecting rough usage by clients. Check for incorrect usage of a class using `assert(...)`, `guarantee(...)`, `ShouldNotReachHere()` and comments wherever needed. Performance is almost never a reason to omit asserts.

- When possible, design as if for reusability. This forces a clear design of the class's externals, and clean hiding of its internals.

- Initialize all variables and data structures to a known state. If a class has a constructor, initialize it there.

- Do no optimization before its time. Prove the need to optimize.

- When you must defactor to optimize, preserve as much structure as possible. If you must hand-inline some name, label the local copy with the original name.

- If you need to use a hidden detail (e.g., a structure offset), name it (as a constant or function) in the class that owns it.

- Don't use the Copy and Paste keys to replicate more than a couple lines of code. Name what you must repeat.

- If a class needs a member function to change a user-visible attribute, the change should be done with a "setter" accessor matched to the simple "getter".

#### Conventions for Lock-free Code

Sometimes variables are accessed concurrently without appropriate synchronization context, such as a held mutex or at a safepoint. In such cases the variable should be declared `volatile` and it should NOT be accessed as a normal C++ lvalue. Rather, access should be performed via functions from `Atomic`, such as `Atomic::load`, `Atomic::store`, etc.

This special formulation makes it more clear to maintainers that the variable is accessed concurrently in a lock-free manner.

### Source Files

- All source files must have a globally unique basename. The build system depends on this uniqueness.

- Keep the include lines within a section alphabetically sorted by their lowercase value. If an include must be out of order for correctness, suffix with it a comment such as `// do not reorder`. Source code processing tools can also use this hint.

- Put conditional inclusions (`#if ...`) at the end of the section of HotSpot include lines. This also applies to macro-expanded includes of platform dependent files.

- Put system includes in a section after the HotSpot include lines with a blank line separating the two sections.

- Do not put non-trivial function implementations in .hpp files. If the implementation depends on other .hpp files, put it in a .cpp or a .inline.hpp file.

- .inline.hpp files should only be included in .cpp or .inline.hpp files.

- All .inline.hpp files should include their corresponding .hpp file as the first include line with a blank line separating it from the rest of the include lines. Declarations needed by other files should be put in the .hpp file, and not in the .inline.hpp file. This rule exists to resolve problems with circular dependencies between .inline.hpp files.

- Do not include a .hpp file if the corresponding .inline.hpp file is included.

- Use include guards for .hpp and .inline.hpp files. The name of the defined guard should be derived from the full search path of the file relative to the hotspot source directory. The guard should be all upper case with all paths separators and periods replaced by underscores.

- Some build configurations use precompiled headers to speed up the build times. The precompiled headers are included in the precompiled.hpp file. Note that precompiled.hpp is just a build time optimization, so don't rely on it to resolve include problems.

### JTReg Tests

- JTReg tests should have meaningful names.

- JTReg tests associated with specific bugs should be tagged with the `@bug` keyword in the test description.

- JTReg tests should be organized by component or feature under `test/`, in a directory hierarchy that generally follows that of the `src/` directory. There may be additional subdirectories to further categorize tests by feature. This structure makes it easy to run a collection of tests associated with a specific feature by specifying the associated directory as the source of the tests to run.

  - Some (older) tests use the associated bug number in the directory name, the test name, or both. That naming style should no longer be used, with existing tests using that style being candidates for migration.

### Naming

- The length of a name may be correlated to the size of its scope. In particular, short names (even single letter names) may be fine in a small scope, but are usually inappropriate for larger scopes.

- Prefer whole words rather than abbreviations, unless the abbreviation is more widely used than the long form in the code's domain.

- Choose names consistently. Do not introduce spurious variations. Abbreviate corresponding terms to a consistent length.

- Global names must be unique, to avoid One Definition Rule (ODR) violations. A common prefixing scheme for related global names is often used. (This is instead of using namespaces, which are mostly avoided in HotSpot.)

- Don't give two names to the semantically same thing. But use different names for semantically different things, even if they are representationally the same. (So use meaningful `typedef` or template alias names where appropriate.)

- When choosing names, avoid categorical nouns like "variable", "field", "parameter", "value", and verbs like "compute", "get". (`storeValue(int param)` is bad.)

- Type names and global names should use mixed-case with the first letter of each word capitalized (`FooBar`).

- Embedded abbreviations in otherwise mixed-case names are usually capitalized entirely rather than being treated as a single word with only the initial letter capitalized, e.g. "HTML" rather than "Html".

- Function and local variable names use lowercase with words separated by a single underscore (`foo_bar`).

- Class data member names have a leading underscore, and use lowercase with words separated by a single underscore (`_foo_bar`).

- Constant names may be upper-case or mixed-case, according to historical necessity. (Note: There are many examples of constants with lowercase names.)

- Constant names should follow an existing pattern, and must have a distinct appearance from other names in related APIs.

- Class and type names should be noun phrases. Consider an "er" suffix for a class that represents an action.

- Function names should be verb phrases that reflect changes of state known to a class's user, or else noun phrases if they cause no change of state visible to the class's user.

- Getter accessor names are noun phrases, with no "`get_`" noise word. Boolean getters can also begin with "`is_`" or "`has_`". Member function for reading data members usually have the same name as the data member, exclusive of the leading underscore.

- Setter accessor names prepend "`set_`" to the getter name.

- Other member function names are verb phrases, as if commands to the receiver.

- Avoid leading underscores (as "`_oop`") except in cases required above. (Names with leading underscores can cause portability problems.)

### Commenting

- Clearly comment subtle fixes.

- Clearly comment tricky classes and functions.

- If you have to choose between commenting code and writing wiki content, comment the code. Link from the wiki to the source file if it makes sense.

- As a general rule don't add bug numbers to comments (they would soon overwhelm the code). But if the bug report contains significant information that can't reasonably be added as a comment, then refer to the bug report.

- Personal names are discouraged in the source code, which is a team product.

### Macros

- You can almost always use an inline function or class instead of a macro. Use a macro only when you really need it.

- Templates may be preferable to multi-line macros. (There may be subtle performance effects with templates on some platforms; revert to macros if absolutely necessary.)

- `#ifdef`s should not be used to introduce platform-specific code into shared code (except for `_LP64`). They must be used to manage header files, in the pattern found at the top of every source file. They should be used mainly for major build features, including `PRODUCT`, `ASSERT`, `_LP64`, `INCLUDE_SERIALGC`, `COMPILER1`, etc.

- For build features such as `PRODUCT`, use `#ifdef PRODUCT` for multiple-line inclusions or exclusions.

- For short inclusions or exclusions based on build features, use macros like `PRODUCT_ONLY` and `NOT_PRODUCT`. But avoid using them with multiple-line arguments, since debuggers do not handle that well.

- Use `CATCH`, `THROW`, etc. for HotSpot-specific exception processing.

### Whitespace

- In general, don't change whitespace unless it improves readability or consistency. Gratuitous whitespace changes will make integrations and backports more difficult.

- Use One-True-Brace-Style. The opening brace for a function or class is normally at the end of the line; it is sometimes moved to the beginning of the next line for emphasis. Substatements are enclosed in braces, even if there is only a single statement. Extremely simple one-line statements may drop braces around a substatement.

- Indentation levels are two columns.

- There is no hard line length limit. That said, bear in mind that excessively long lines can cause difficulties. Some people like to have multiple side-by-side windows in their editors, and long lines may force them to choose among unpleasant options. They can use wide windows, reducing the number that can fit across the screen, and wasting a lot of screen real estate because most lines are not that long. Alternatively, they can have more windows across the screen, with long lines wrapping (or worse, requiring scrolling to see in their entirety), which is harder to read. Similar issues exist for side-by-side code reviews.

- Tabs are not allowed in code. Set your editor accordingly.
  (Emacs: `(setq-default indent-tabs-mode nil)`.)

- Use good taste to break lines and align corresponding tokens on adjacent lines.

- Use spaces around operators, especially comparisons and assignments. (Relaxable for boolean expressions and high-precedence operators in classic math-style formulas.)

- Put spaces on both sides of control flow keywords `if`, `else`, `for`, `switch`, etc. Don't add spaces around the associated control expressions. Examples:

      while (test_foo(args...)) {   // Yes
      while(test_foo(args...)) {    // No, missing space after while
      while ( test_foo(args...) ) { // No, excess spaces around control

- Use extra parentheses in expressions whenever operator precedence seems doubtful. Always use parentheses in shift/mask expressions (`<<`, `&`, `|`). Don't add whitespace immediately inside parentheses.

- Use more spaces and blank lines between larger constructs, such as classes or function definitions.

- If the surrounding code has any sort of vertical organization, adjust new lines horizontally to be consistent with that organization. (E.g., trailing backslashes on long macro definitions often align.)

### Avoid implicit conversions to bool

- Use `bool` for boolean values.
- Do not use ints or pointers as (implicit) booleans with `&&`, `||`, `if`, `while`. Instead, compare explicitly, i.e. `if (x != 0)` or `if (ptr != nullptr)`, etc.
- Do not use non-boolean declarations in condition forms, i.e. don't use `if (T v = value) { ... }`. But see Enhanced selection statements.

### Miscellaneous

- Use the Resource Acquisition Is Initialization (RAII) design pattern to manage bracketed critical sections. See class `ResourceMark` for an example.

- Use functions from globalDefinitions.hpp and related files when performing bitwise operations on integers. Do not code directly as C operators, unless they are extremely simple. (Examples: `align_up`, `is_power_of_2`, `exact_log2`.)

- Use arrays with abstractions supporting range checks.

- Always enumerate all cases in a switch statement or provide a default case. It is ok to have an empty default with comment.

## Use of C++ Features

HotSpot was originally written in a subset of the C++98/03 language. More recently, support for C++17 is provided, though again, HotSpot only uses a subset. (Backports to JDK versions lacking support for more recent Standards must of course stick with the original C++98/03 subset.)

This section describes that subset. Features from the C++98/03 language may be used unless explicitly forbidden here. Features from C++11, C++14, and C++17 may be explicitly permitted or explicitly forbidden, and discussed accordingly here. There is a third category, undecided features, about which HotSpot developers have not yet reached a consensus, or perhaps have not discussed at all. Use of these features is also forbidden.

(The use of some features may not be immediately obvious and may slip in anyway, since the compiler will accept them. The code review process is the main defense against this.)

Some features are discussed in their own subsection, typically to provide more extensive discussion or rationale for limitations. Features that don't have their own subsection are listed in omnibus feature sections for permitted, forbidden, and undecided features.

Lists of new features for C++11, C++14, and C++17, along with links to their descriptions, can be found in the online documentation for some of the compilers and libraries. The C++17 Standard is the definitive description.

- C++ Standards Support in GCC
- C++ Support in Clang
- Visual C++ Language Conformance
- libstdc++ Status
- libc++ Status

As a rule of thumb, permitting features which simplify writing code and, especially, reading code, is encouraged.

Similar discussions for some other projects:

- Google C++ Style Guide — Currently (2020) targeting C++17.

- C++11 and C++14 use in Chromium — Categorizes features as allowed, banned, or to be discussed.

- llvm Coding Standards — Currently (2020) targeting C++14.

- Using C++ in Mozilla code — C++17 support is required for recent versions (2020).

### Error Handling

Do not use exceptions. Exceptions are disabled by the build configuration for some platforms.

Rationale: There is significant concern over the performance cost of exceptions and their usage model and implications for maintainable code. That's not just a matter of history that has been fixed; there remain questions and problems even today (2019). See, for example, Zero cost deterministic exceptions. Because of this, HotSpot has always used a build configuration that disables exceptions where that is available. As a result, HotSpot code uses error handling mechanisms such as two-phase construction, factory functions, returning error codes, and immediate termination. Even if the cost of exceptions were not a concern, the existing body of code was not written with exception safety in mind. Making HotSpot exception safe would be a very large undertaking.

In addition to the usual alternatives to exceptions, HotSpot provides its own exception mechanism. This is based on a set of macros defined in utilities/exceptions.hpp.

### RTTI (Runtime Type Information)

Do not use Runtime Type Information (RTTI). RTTI is disabled by the build configuration for some platforms. Among other things, this means `dynamic_cast` cannot be used.

Rationale: Other than to implement exceptions (which HotSpot doesn't use), most potential uses of RTTI are better done via virtual functions. Some of the remainder can be replaced by bespoke mechanisms. The cost of the additional runtime data structures needed to support RTTI are deemed not worthwhile, given the alternatives.

### Memory Allocation

Do not use the standard global allocation and deallocation functions (global `operator new` and related functions), other than the non-allocating forms of those functions. Use of these functions by HotSpot code is disabled for some platforms.

Rationale: HotSpot often uses "resource" or "arena" allocation. Even where heap allocation is used, the standard global functions are avoided in favor of wrappers around `malloc` and `free` that support the JVM's Native Memory Tracking (NMT) feature. Typically, uses of the global `operator new` are inadvertent and therefore often associated with memory leaks.

Native memory allocation failures are often treated as non-recoverable. The place where "out of memory" is (first) detected may be an innocent bystander, unrelated to the actual culprit.

### Class Inheritance

Use public single inheritance.

Prefer composition rather than non-public inheritance.

Restrict inheritance to the "is-a" case; use composition rather than non-is-a related inheritance.

Avoid multiple inheritance. Never use virtual inheritance.

### Namespaces

Avoid using namespaces. HotSpot code normally uses "all static" classes rather than namespaces for grouping. An "all static" class is not instantiable, has only static members, and is normally derived (possibly indirectly) from the helper class `AllStatic`.

Benefits of using such classes include:

- Provides access control for members, which is unavailable with namespaces.

- Avoids Argument Dependent Lookup (ADL).

- Closed for additional members. Namespaces allow names to be added in multiple contexts, making it harder to see the complete API.

Namespaces should be used only in cases where one of those "benefits" is actually a hindrance.

In particular, don't use anonymous namespaces. They seem like they should be useful, and indeed have some real benefits for naming and generated code size on some platforms. Unfortunately, debuggers don't seem to like them at all.

https://groups.google.com/forum/#!topic/mozilla.dev.platform/KsaG3lEEaRM
Suggests Visual Studio debugger might not be able to refer to anonymous namespace symbols, so can't set breakpoints in them. Though the discussion seems to go back and forth on that.

https://firefox-source-docs.mozilla.org/code-quality/coding-style/coding_style_cpp.html
Search for "Anonymous namespaces" Suggests preferring "static" to anonymous namespaces where applicable, because of poor debugger support for anonymous namespaces.

https://sourceware.org/bugzilla/show_bug.cgi?id=16874
Bug for similar gdb problems.

### C++ Standard Library

Only curated parts of the C++ Standard Library may be used by HotSpot code.

Functions that may throw exceptions must not be used. This is in accordance with the HotSpot policy of not using exceptions.

Also in accordance with HotSpot policy, the standard global allocator must not be used. This means that uses of standard container classes cannot presently be used, as doing so requires specialization on some allocator type that is integrated with the existing HotSpot allocation mechanisms. (Such allocators may be provided in the future.)

Standard Library identifiers should usually be fully qualified; `using` directives must not be used to bring Standard Library identifiers into scope just to remove the need for namespace qualification. Requiring qualification makes it easy to distinguish between references to external libraries and code that is part of HotSpot.

As with language features, Standard Library facilities are either permitted, explicitly forbidden, or undecided (and so implicitly forbidden).

Most HotSpot code should not directly `#include` C++ Standard Library headers. HotSpot provides a set of wrapper headers for the Standard Library headers containing permitted definitions. These wrappers are in the `cppstdlib` directory, with the same name as the corresponding Standard Library header and a `.hpp` extension. These wrappers provide a place for any additional code (some of which may be platform-specific) needed to support HotSpot usage.

These wrappers also provide a place to document HotSpot usage, including any restrictions. The set of wrappers and the usage documentation should be considered part of HotSpot style. Any changes are subject to the same process as applies to this document. (For historical reasons, there may be many direct inclusions of some C++ Standard Library headers.)

Historically, HotSpot has mostly avoided use of the Standard Library.

(It used to be impossible to use most of it in shared code, because the build configuration for Solaris with Solaris Studio made all but a couple of pieces inaccessible. Support for header-only parts was added in mid-2017. Support for Solaris was removed in 2020.)

Some reasons for this include

- Exceptions. Perhaps the largest core issue with adopting the use of Standard Library facilities is exceptions. HotSpot does not use exceptions and, for platforms which allow doing so, builds with them turned off. Many Standard Library facilities implicitly or explicitly use exceptions. On the other hand, many don't, and can be used without concern for this issue. Others may have a useful subset that doesn't use exceptions.

- `assert`. An issue that is quickly encountered is the `assert` macro name collision (JDK-8007770). (Not all Standard Library implementations use `assert` in header files, but some do.) HotSpot provides a mechanism for addressing this, by establishing a scope around the include of a library header where the HotSpot `assert` macro is suppressed. One of the reasons for using wrapper headers rather than directly including standard headers is to provide a central place to deal with this issue for each header.

- Memory allocation. HotSpot requires explicit control over where allocations occur. The C++98/03 `std::allocator` class is too limited to support our usage. But changes to the allocator concept in more recent Standards removed some of the limitations, supporting stateful allocators. HotSpot may, in the future, provide standard-conforming allocators that are integrated with HotSpot's existing allocation mechanisms.

- Implementation vagaries. Bugs, or simply different implementation choices, can lead to different behaviors among the various Standard Libraries we need to deal with. But only selected parts of the Standard Library are being permitted, and one of the selection criteria is maturity. Some of these facilities are among the most heavily tested and used C++ codes that exist.

- Inconsistent naming conventions. HotSpot and the C++ Standard use different naming conventions. The coexistence of those different conventions might appear jarring and reduce readability. However, experience in some other code bases suggests this isn't a significant problem, so long as Standard Library names are namespace-qualified. It is tempting to bring the Standard Library names into scope via a `using std;` directive. Doing so makes writing code using those names easier, since the qualifiers don't need to be included. But there are several reasons not to do that.

  - There is a risk of future name collisions. Additional Standard Library headers may be included, adding to the list of names being used. Also, future versions of the Standard Library may add currently unknown names to the headers already being included.

  - It may harm readability. Code where this is relevant is a mixture of the local HotSpot naming conventions and the Standard Library's (or other 3rd-party library's) naming conventions. With only unqualified names, any distinctions from the naming conventions for the different code sources are lost. Instead one may end up with an undifferentiated mess, where it's not obvious whether an identifier is from local code that is inconsistent with HotSpot style (and there's a regretable amount of that for historical reasons), or is following some other convention. Having the qualifiers disambiguates that.

  - It can be helpful to know, at a glance, whether the definition is in HotSpot or elsewhere, for purposes of looking up the definition or documentation.

### Type Deduction

Use type deduction only if it makes the code clearer or safer. Do not use it merely to avoid the inconvenience of writing an explicit type, unless that type is itself difficult to write. An example of the latter is a function template return type that depends on template parameters in a non-trivial way.

There are several contexts where types are deduced.

- Function argument deduction. This is always permitted, and indeed encouraged. It is nearly always better to allow the type of a function template argument to be deduced rather than explicitly specified.

- `auto` variable declarations (n1984)
  For local variables, this can be used to make the code clearer by eliminating type information that is obvious or irrelevant. Excessive use can make code much harder to understand.

- `auto` for non-type template parameters (p0127r2)
  `auto` may be used as a placeholder for the type of a non-type template parameter. The type is deduced from the value provided in a template instantiation.

* Function return type deduction (n3638)
Only use if the function body has a very small number of `return` statements, and generally relatively little other code.

- Class template argument deduction (n3602, p0091r3)
  The template arguments of a class template may be deduced from the arguments to a constructor. This is similar to ordinary function argument deduction, though partial deduction with only some template arguments explicitly provided is not permitted for class template argument deduction. Deduction guides may be used to provide additional control over the deduction. As with `auto` variable declarations, excessive use can make code harder to understand, because explicit type information is lacking. But it can also remove the need to be explicit about types that are either obvious, or that are very hard to write. For example, these allow the addition of a scope-guard mechanism with nice syntax; something like this

      ScopeGuard guard{[&]{ ... cleanup code ... }};

- Also see lambda expressions.

- `decltype(auto)` should be avoided, whether for variables, for non-type template parameters, or for function return types. There are subtle and complex differences between this placeholder type and `auto`. Any use would need very careful explanation.

### Expression SFINAE

Substitution Failure Is Not An Error (SFINAE) is a template metaprogramming technique that makes use of template parameter substitution failures to make compile-time decisions.

C++11 relaxed the rules for what constitutes a hard-error when attempting to substitute template parameters with template arguments, making most deduction errors be substitution errors; see (n2634). This makes SFINAE more powerful and easier to use. However, the implementation complexity for this change is significant, and this seems to be a place where obscure corner-case bugs in various compilers can be found. So while this feature can (and indeed should) be used (and would be difficult to avoid), caution should be used when pushing to extremes.

Here are a few closely related example bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95468
https://developercommunity.visualstudio.com/content/problem/396562/sizeof-deduced-type-is-sometimes-not-a-constant-ex.html

### Trailing return type syntax for functions

A function's return type may be specified after the parameters and qualifiers (n2541). In such a declaration the normal return type is `auto` and the return type is indicated by `->` followed by the type. Although both use `auto` in the "normal" leading return type position, this differs from function return type deduction, in that the return type is explicit rather than deduced, but specified in a trailing position.

Use of trailing return types is permitted. However, the normal, leading position for the return type is preferred. A trailing return type should only be used where it provides some benefit. Such benefits usually arise because a trailing return type is in a different scope than a leading return type.

- If the function identifier is a nested name specifier, then the trailing return type occurs in the nested scope. This may permit simpler naming in the return type because of the different name lookup context.

- The trailing return type is in the scope of the parameters, making their types accessible via `decltype`. For example

    template<typename T, typename U> auto add(T t, U u) -> decltype(t + u);

rather than

    template<typename T, typename U> decltype((*(T*)0) + (*(U*)0)) add(T t, U u);

- Complex calculated leading return types may obscure the normal syntactic boundaries, making it more difficult for a reader to find the function name and parameters. This is particularly common in cases where the return type is being used for SFINAE. A trailing return type may be preferable in such situations.

### Non-type template parameter values

C++17 extended the arguments permitted for non-type template parameters (n4268). The kinds of values (the parameter types) aren't changed. However, the values can now be the result of arbitrary constant expressions (with a few restrictions on the result), rather than a much more limited and restrictive set of expressions. In particular, the argument for a pointer or reference type parameter can now be the result of a constexpr function.

### enum

Where appropriate, scoped-enums should be used. (n2347)

Use of unscoped-enums is permitted, though ordinary constants may be preferable when the automatic initializer feature isn't used.

The underlying type (the enum-base) of an unscoped enum type should always be specified explicitly. When unspecified, the underlying type is dependent on the range of the enumerator values and the platform.

The underlying type of a scoped-enum should also be specified explicitly if conversions may be applied to values of that type.

Due to bugs in certain (very old) compilers, there is widespread use of enums and avoidance of in-class initialization of static integral constant members. Compilers having such bugs are no longer supported. Except where an enum is semantically appropriate, new code should use integral constants.

### alignas

Alignment-specifiers (`alignas` n2341) are permitted, with restrictions.

Alignment-specifiers are permitted when the requested alignment is a fundamental alignment (not greater than `alignof(std::max_align_t)` C++14 3.11/2).

Alignment-specifiers with an extended alignment (greater than `alignof(std::max_align_t)` C++14 3.11/3) may only be used to align variables with static or automatic storage duration (C++14 3.7.1, 3.7.3). As a consequence, over-aligned types are forbidden; this may change if HotSpot updates to using C++17 or later (p0035r4).

Large extended alignments should be avoided, particularly for stack allocated objects. What is a large value may depend on the platform and configuration. There may also be hard limits for some platforms.

An alignment-specifier must always be applied to a definition (C++14 10.6.2/6). (C++ allows an alignment-specifier to optionally also be applied to a declaration, so long as the definition has equivalent alignment. There isn't any known benefit from duplicating the alignment in a non-definition declaration, so such duplication should be avoided in HotSpot code.)

Enumerations are forbidden from having alignment-specifiers. Aligned enumerations were originally permitted but insufficiently specified, and were later (C++20) removed (CWG 2354). Permitting such usage in HotSpot now would just cause problems in the future.

Alignment-specifiers are forbidden in `typedef` and alias-declarations. This may work or may have worked in some versions of some compilers, but was later (C++14) explicitly disallowed (CWG 1437).

The HotSpot macro `ATTRIBUTE_ALIGNED` provides similar capabilities for platforms that define it. This macro predates the use by HotSpot of C++ versions providing `alignas`. New code should use `alignas`.

### thread_local

Avoid use of `thread_local` (n2659); and instead, use the HotSpot macro `THREAD_LOCAL`, for which the initializer must be a constant expression. When `thread_local` must be used, use the Hotspot macro `APPROVED_CPP_THREAD_LOCAL` to indicate that the use has been given appropriate consideration.

As was discussed in the review for JDK-8230877, `thread_local` allows dynamic initialization and destruction semantics. However, that support requires a run-time penalty for references to non-function-local `thread_local` variables defined in a different translation unit, even if they don't need dynamic initialization. Dynamic initialization and destruction of non-local `thread_local` variables also has the same ordering problems as for ordinary non-local variables. So we avoid use of `thread_local` in general, limiting its use to only those cases where dynamic initialization or destruction are essential. See JDK-8282469 for further discussion.

### nullptr

Use `nullptr` (n2431) rather than `NULL`. See the paper for reasons to avoid `NULL`.

Don't use (constant expression or literal) 0 for pointers. Note that C++14 removed non-literal 0 constants from null pointer constants, though some compilers continue to treat them as such.

### <atomic>

Do not use facilities provided by the `<atomic>` header (n2427), (n2752); instead, use the HotSpot `Atomic` class and related facilities.

Atomic operations in HotSpot code must have semantics which are consistent with those provided by the JDK's compilers for Java. There are platform-specific implementation choices that a C++ compiler might make or change that are outside the scope of the C++ Standard, and might differ from what the Java compilers implement.

In addition, HotSpot `Atomic` has a concept of "conservative" memory ordering, which may differ from (may be stronger than) sequentially consistent. There are algorithms in HotSpot that are believed to rely on that ordering.

### Variable Templates and Inline Variables

The use of variable templates (including static data member templates) (N3651) is permitted. They provide parameterized variables and constants in a simple and direct form, instead of requiring the use of various workarounds.

Variables with static storage duration and variable templates may be declared `inline` (p0386r2), and this usage is permitted. This has similar effects as for declaring a function inline: it can be defined, identically, in multiple translation units, must be defined in every translation unit in which it is ODR used, and the behavior of the program is as if there is exactly one variable.

Declaring a variable inline allows the complete definition to be in a header file, rather than having a declaration in a header and the definition in a .cpp file. The guidance on initialization of such variables still applies. Inline variables with dynamic initializations can make initialization order problems worse. The few ordering constraints that exist for non-inline variables don't apply, as there isn't a single program-designated translation unit containing the definition.

A `constexpr` static data member or static data member template is implicitly `inline`. As a consequence, an ODR use of such a member doesn't require a definition in some .cpp file. (This is a change from pre-C++17. Beginning with C++17, such a definition is considered a duplicate definition, and is deprecated.)

Declaring a `thread_local` variable template or `inline` variable is forbidden in HotSpot code. The use of `thread_local` is already heavily restricted.

### Initializing variables with static storage duration

Variables with static storage duration and dynamic initialization C++14 3.6.2). should be avoided, unless an implementation is permitted to perform the initialization as a static initialization. The order in which dynamic initializations occur is incompletely specified. Initialization order problems can be difficult to deal with and lead to surprises.

Variables with static storage duration and non-trivial destructors should be avoided. HotSpot doesn't generally try to cleanup on exit, and running destructors at exit can lead to problems.

Some of the approaches used in HotSpot to avoid dynamic initialization include:

- Use the `DeferredStatic<T>` class template. Add a call to its initialization function at an appropriate place during VM initialization. The underlying object is never destroyed.

- For objects of class type, use a variable whose value is a pointer to the class, initialized to `nullptr`. Provide an initialization function that sets the variable to a dynamically allocated object. Add a call to that function at an appropriate place during VM initialization. Such objects are usually never destroyed.

### Uniform Initialization

The use of uniform initialization (n2672), also known as brace initialization, is permitted.

Some relevant sections from cppreference.com:

- initialization
- value initialization
- direct initialization
- list initialization
- aggregate initialization

Although related, the use of `std::initializer_list` remains forbidden, as part of the avoidance of the C++ Standard Library in HotSpot code.

### Mandatory Copy Elision

Copy elision (or here) is a compiler optimization used to avoid potentially expensive copies in certain situations. It is critical to making practical the performance of return by value or pass by value. It is also unusual in not following the as-if rule for optimizations - copy elision can be applied even if doing so bypasses side-effects of copying/moving the object. The C++ standard explicitly permits this.

However, because it's an optional optimization, the relevant copy/move constructor must be available and accessible, in case the compiler chooses to not apply the optimization even in a situation where permitted.

C++17 changed some cases of copy elision so that there is never a copy/move in these cases (p0135r1). The interesting cases involve a function that returns an unnamed temporary object, and constructors. In such cases the object being initialized from the temporary is always direct initialized, with no copy/move ever involved; see RVO and more specifically URVO.

Since this is now standard behavior it can't be avoided in the covered situations. This could change the behavior of code that relied on side effects by constructors, but that's both uncommon and was already problematic because of the previous optional copy elision. But HotSpot code can, and should, explicitly take advantage of this newly required behavior where it makes sense to do so.

For example, it may be beneficial to delay construction of the result of a function until the return statement, rather than having a local variable that is modified into the desired state and then returned. (Though NRVO may apply in that case.)

It is also now possible to define a factory function for a class that is neither movable nor copyable, if it can be written in a way that makes use of this feature.

### Local Function Objects

- Local function objects, including lambda expressions, may be used.
- Lambda expressions must only be used as a downward value.
- Prefer `[&]` as the capture list of a lambda expression.
- Return type deduction for lambda expressions is permitted, and indeed encouraged.
- An empty parameter list for a lambda expression may be elided.
- A lambda expression must not be `mutable`.
- Generic lambda expressions are permitted.
- Lambda expressions should be relatively simple.
- Anonymous lambda expressions should not overly clutter the enclosing expression.
- An anonymous lambda expression must not be directly invoked.
- Bind expressions are forbidden.

Single-use function objects can be defined locally within a function, directly at the point of use. This is an alternative to having a function or function object class defined at class or namespace scope.

This usage was somewhat limited by C++03, which does not permit such a class to be used as a template parameter. That restriction was removed by C++11 (n2657). Use of this feature is permitted.

Many HotSpot protocols involve "function-like" objects that involve some named member function rather than a call operator. For example, a function that performs some action on all threads might be written as

    void do_something() {
      struct DoSomething : public ThreadClosure {
        virtual void do_thread(Thread* t) {
          ... do something with t ...
        }
      } closure;
      Threads::threads_do(&closure);
    }

HotSpot code has historically usually placed the DoSomething class at namespace (or sometimes class) scope. This separates the function's code from its use, often to the detriment of readability. It requires giving the class a globally unique name (if at namespace scope). It also loses the information that the class is intended for use in exactly one place, and does not have any subclasses. (However, the latter can now be indicated by declaring it `final`.) Often, for simplicity, a local class will skip things like access control and accessor functions, giving the enclosing function direct access to the implementation and eliminating some boilerplate that might be provided if the class is in some outer (more accessible) scope. On the other hand, if there is a lot of surrounding code in the function body or the local class is of significant size, defining it locally can increase clutter and reduce readability.

C++11 added lambda expressions as a new way to write a function object. Simple lambda expressions can be significantly more concise than a function object, eliminating a lot of boiler-plate. On the other hand, a complex lambda expression may not provide much, if any, readability benefit compared to an ordinary function object. Also, while a lambda can encapsulate a call to a "function-like" object, it cannot be used in place of such.

A common use for local functions is as one-use RAII objects. The amount of boilerplate for a function object class (local or not) makes such usage somewhat clumsy and verbose. But with the help of a small amount of supporting utility code, lambdas work particularly well for this use case.

Another use for local functions is partial application. Again here, lambdas are typically much simpler and less verbose than function object classes.

A lambda is a constexpr function if either the parameter declaration clause is followed by `constexpr`, or it satisfies the requirements for a constexpr function (p0170r1). Thus, using a lambda to package up some computation doesn't incur unnecessary overhead or prevent use in a context required to be compile-time evaluated (such as an array size).

Because of these benefits, lambda expressions are permitted in HotSpot code, with some restrictions and usage guidance. An anonymous lambda is one which is passed directly as an argument. A named lambda is the value of a variable, which is its name.

Lambda expressions should only be passed downward. In particular, a lambda should not be returned from a function or stored in a global variable, whether directly or as the value of a member of some other object. Lambda capture is syntactically subtle (by design), and propagating a lambda in such ways can easily pass references to captured values to places where they are no longer valid. In particular, members of the enclosing `this` object are effectively captured by reference, even if the default capture is by-value. For such uses-cases a function object class should be used to make the desired value capturing and propagation explicit.

Limiting the capture list to `[&]` (implicitly capture by reference) is a simplifying restriction that still provides good support for HotSpot usage, while reducing the cases a reader must recognize and understand.

- Many common lambda uses require reference capture. Not permitting it would substantially reduce the utility of lambdas.

- Referential transparency. Implicit reference capture makes variable references in the lambda body have the same meaning they would have in the enclosing code. There isn't a semantic barrier across which the meaning of a variable changes.

- Explicit reference capture introduces significant clutter, especially when lambda expressions are relatively small and simple, as they should be in HotSpot code.

- There are a number of reasons why by-value capture might be used, but for the most part they don't apply to HotSpot code, given other usage restrictions.

  - A primary use-case for by-value capture is to support escaping uses, where values captured by-reference might become invalid. That use-case doesn't apply if only downward lambdas are used.

  - By-value capture can also make a lambda-local copy for mutation, which requires making the lambda `mutable`; see below.

  - By-value capture might be viewed as an optimization, avoiding any overhead for reference capture of cheap to copy values. But the compiler can often eliminate any such overhead.

  - By-value capture by a non-`mutable` lambda makes the captured values const, preventing any modification by the lambda and making the captured value unaffected by modifications to the outer variable. But this only applies to captured auto variables, not member variables, and is inconsistent with referential transparency.

- By-value capture of `this` (using a capture list like `[*this]` (p0018r3)) is also not permitted. One of the motivating use-cases is when the lifetime of the lambda exceeds the lifetime of the object for the containing member function. That is, we have an upward lambda that is capturing `this` of the enclosing method. But again, that use-case doesn't apply if only downward lambdas are used. Another use-case is when we simply want the lambda to be operating on a copy of `this` for some reason. This is sufficiently uncommon that it can be handled by manual copying, so readers don't need to understand this rare syntax.

- Non-capturing lambdas (with an empty capture list - `[]`) have limited utility. There are cases where no captures are required (pure functions, for example), but if the function is small and simple then that's obvious anyway.

- Capture initializers (a C++14 feature - N3649) are not permitted. Capture initializers inherently increase the complexity of the capture list, and provide little benefit over an additional in-scope local variable.

- The use of `mutable` lambda expressions is forbidden because there don't seem to be many, if any, good use-cases for them in HotSpot. A lambda expression needs to be mutable in order to modify a by-value captured value. But with only downward lambdas, such usage seems likely to be rare and complicated. It is better to use a function object class in any such cases that arise, rather than requiring all HotSpot developers to understand this relatively obscure feature.

While it is possible to directly invoke an anonymous lambda expression, that feature should not be used, as such a form can be confusing to readers. Instead, name the lambda and call it by name.

Some reasons to prefer a named lambda instead of an anonymous lambda are

- The body contains non-trivial control flow or declarations or other nested constructs.

- Its role in an argument list is hard to guess without examining the function declaration. Give it a name that indicates its purpose.

- It has an unusual capture list.

- It has a complex explicit return type or parameter types.

Lambda expressions, and particularly anonymous lambda expressions, should be simple and compact. One-liners are good. Anonymous lambdas should usually be limited to a couple lines of body code. More complex lambdas should be named. A named lambda should not clutter the enclosing function and make it long and complex; do continue to break up large functions via the use of separate helper functions.

An anonymous lambda expression should either be a one-liner in a one-line expression, or isolated in its own set of lines. Don't place part of a lambda expression on the same line as other arguments to a function. The body of a multi-line lambda argument should be indented from the start of the capture list, as if that were the start of an ordinary function definition. The body of a multi-line named lambda should be indented one step from the variable's indentation.

Some examples:

1. `foo([&] { ++counter; });`

2. `foo(x, [&] { ++counter; });`

3. `foo([&] { if (predicate) ++counter; });`

4. `foo([&] { auto tmp = process(x); tmp.f(); return tmp.g(); })`

5. Separate one-line lambda from other arguments:

       foo(c.begin(), c.end(),
           [&] (const X& x) { do_something(x); return x.value(); });

6. Indentation for multi-line lambda:

       c.do_entries([&] (const X& x) {
                      do_something(x, a);
                      do_something1(x, b);
                      do_something2(x, c);
                    });

7. Separate multi-line lambda from other arguments:

       foo(c.begin(), c.end(),
           [&] (const X& x) {
             do_something(x, a);
             do_something1(x, b);
             do_something2(x, c);
           });

8. Multi-line named lambda:

       auto do_entry = [&] (const X& x) {
         do_something(x, a);
         do_something1(x, b);
         do_something2(x, c);
       };

Item 4, and especially items 6 and 7, are pushing the simplicity limits for anonymous lambdas. Item 6 might be better written using a named lambda:

    c.do_entries(do_entry);

Note that C++11 also added bind expressions as a way to write a function object for partial application, using `std::bind` and related facilities from the Standard Library. `std::bind` generalizes and replaces some of the binders from C++03. Bind expressions are not permitted in HotSpot code. They don't provide enough benefit over lambdas or local function classes in the cases where bind expressions are applicable to warrant the introduction of yet another mechanism in this space into HotSpot code.

References:

- Local and unnamed types as template parameters (n2657)
- New wording for C++0x lambdas (n2927)
- Generalized lambda capture (init-capture) (N3648)
- Generic (polymorphic) lambda expressions (N3649)

References from C++17

- Wording for constexpr lambda (p0170r1)
- Lambda capture of *this by Value (p0018r3)

References from C++20

- Allow lambda capture [=, this] (p0409r2)
- Familiar template syntax for generic lambdas (p0428r2)
- Simplifying implicit lambda capture (p0588r1)
- Default constructible and assignable stateless lambdas (p0624r2)
- Lambdas in unevaluated contexts (p0315r4)
- Allow pack expansion in lambda init-capture (p0780r2) (p2095r0)
- Deprecate implicit capture of this via [=] (p0806r2)

References from C++23

- Make () more optional for lambdas (p1102r2)

### Inheriting constructors

Do not use inheriting constructors (n2540).

C++11 provides simple syntax allowing a class to inherit the constructors of a base class. Unfortunately there are a number of problems with the original specification, and C++17 contains significant revisions (p0136r1 opens with a list of 8 Core Issues). Although those issues have been addressed, the benefits from this feature are small compared to the complexity. Because of this, HotSpot code must not use inherited constructors.

p0195r0

### Attributes

The use of some attributes (n2761) (listed below) is permitted. (Note that some of the attributes defined in that paper didn't make it into the final specification.)

Attributes are syntactically permitted in a broad set of locations, but specific attributes are only permitted in a subset of those locations. In some cases an attribute that appertains to a given element may be placed in any of several locations with the same meaning. In those cases HotSpot has a preferred location.

- An attribute that appertains to a function is placed at the beginning of the function's declaration, rather than between the function name and the parameter list.

p0068r0 is the initial proposal for the attributes added by C++17.)

Only the following attributes are permitted:

- `[[noreturn]]`
- `[[nodiscard]]` (p0189r1)
- `[[maybe_unused]]` (p0212r1)
- `[[fallthrough]]` (p0188r1)

The following attributes are expressly forbidden:

- `[[carries_dependency]]` - Related to `memory_order_consume`.
- `[[deprecated]]` - Not relevant in HotSpot code.

Direct use of non-standard (and presumably scoped) attributes in shared code is also forbidden. Using such would depend on the C++17 feature that an attribute not recognized by the implementation is ignored (p0283r2). If such an attribute is needed in shared code, the well-established technique of providing an `ATTRIBUTE_XXX` macro with per-compiler definitions (sometimes empty) should be used. Compilers may warn about unrecognized attributes (whether by name or by location), in order to report typos or misuse. Disabling such warnings globally would not be desirable.

The use of `using` directives in attribute lists is also forbidden. (p0028r0) (p0028r4) We don't generally use scoped attributes in attribute lists with other attributes. Rather, uses of scoped attributes (which are implementation defined) are generally hidden behind a portability macro that includes the surrounding brackets.

### noexcept

Use of `noexcept` exception specifications (n3050) are permitted with restrictions described below.

- Only the argument-less form of `noexcept` exception specifications are permitted.
- Allocation functions that may return `nullptr` to indicate allocation failure must be declared `noexcept`.
- All other uses of `noexcept` exception specifications are forbidden.
- `noexcept` expressions are forbidden.
- Dynamic exception specifications are forbidden.

HotSpot is built with exceptions disabled, e.g. compile with `-fno-exceptions` (gcc, clang) or no `/EH` option (MSVC++). So why do we need to consider `noexcept` at all? It's because `noexcept` exception specifications serve two distinct purposes.

The first is to allow the compiler to avoid generating code or data in support of exceptions being thrown by a function. But this is unnecessary, because exceptions are disabled.

The second is to allow the compiler and library code to choose different algorithms, depending on whether some function may throw exceptions. This is only relevant to a certain set of functions.

- Some allocation functions (`operator new` and `operator new[]`) return `nullptr` to indicate allocation failure. If a `new` expression calls such an allocation function, it must check for and handle that possibility. Declaring such a function `noexcept` informs the compiler that `nullptr` is a possible result. If an allocation function is not declared `noexcept` then the compiler may elide that checking and handling for a `new` expression calling that function.

- Certain Standard Library facilities (notably containers) provide different guarantees for some operations (and may choose different algorithms to implement those operations), depending on whether certain functions (constructors, copy/move operations, swap) are nothrow or not. They detect this using type traits that test whether a function is declared `noexcept`. This can have a significant performance impact if, for example, copying is chosen over a potentially throwing move. But this isn't relevant, since HotSpot forbids the use of most Standard Library facilities.

HotSpot code can assume no exceptions will ever be thrown, even from functions not declared `noexcept`. So HotSpot code doesn't ever need to check, either with conditional exception specifications or with `noexcept` expressions.

The exception specification is part of the type of a function (p0012r1. This likely has little impact on HotSpot code, since the use of `noexcept` is expected to be rare.

Dynamic exception specifications were deprecated in C++11. C++17 removed all but `throw()`, with that remaining a deprecated equivalent to `noexcept`.

### Enhanced selection statements

C++17 modified the condition part of `if` and `switch` statements, permitting an init-statement to be included (p0305r1).

Use of this feature is permitted. (However, complex uses may interfere with readability.) Limiting the scope of a variable involved in the condition, while also making the value available to the statement's body, can improve readability. The alternative method of scope-limiting by introducing a nested scope isn't very popular and is rarely used.

This new syntax is in addition to the condition being a declaration with a brace-or-equal-initializer. For an `if` statement this new sytax gains that benefit without violating the long-standing guidance against using implicit conversions to `bool`, which still stands.

For example, uses of Unified Logging sometimes explicitly check whether a `LogTarget` is enabled. Instead of

      LogTarget(...) lt;
      if (lt.is_enabled()) {
        LogStream log(lt);
        ... use log ...
      }
      ... lt is accessible but probably not needed here ...

using this feature one could write

      if (LogTarget(...) lt; lt.is_enabled()) {
        LogStream log(lt);
        ... use log ...
      }

C++17 also added compile-time `if` statements (p0292r2). Use of `if constexpr` is permitted. This feature can replace and (sometimes vastly) simplify many uses of SFINAE. The same declaration and initialization guidance for the condition part apply here as for ordinary `if` statements.

### Expression Evaluation Order

C++17 tightened up the evaluation order for some kinds of subexpressions (p0138r2). Note, however, that the Alternate Evaluation Order for Function Calls alternative in that paper was adopted, rather than the strict left to right order of evaluation for function call arguments that was proposed in the main body of the paper.

The primary purpose of this change seems to be to make certain kinds of call chaining well defined. That's not a style widely used in HotSpot. In general it is better to continue to avoid questions in this area by isolating operations with side effects from other statements. In particular, continue to avoid modifying a value in an expression where it is also used.

### Compatibility with C11

C++17 refers to C11 rather than C99. This means that C11 libraries and functions may be used in HotSpot. There may be limitations because of differing levels of compatibility among various compilers and versions of those compilers.

Note that the C parts of the JDK have been built with C11 selected for some time (JDK-8292008).

### Additional Permitted Features

- `alignof` (n2341)

- `constexpr` (n2235) (n3652)

- Sized deallocation (n3778)

- Variadic templates (n2242) (n2555)

- Static assertions (n1720) (n3928)
  Both the original (C++11) two-argument form and the new (C++17) single-argument form are permitted.

- `decltype` (n2343) (n3276)

- Right angle brackets (n1757)

- Default template arguments for function templates (CWG D226)

- Template aliases (n2258)

- Delegating constructors (n1986)

- Explicit conversion operators (n2437)

- Standard Layout Types (n2342)

- Defaulted and deleted functions (n2346)

- Dynamic initialization and destruction with concurrency (n2660)

- `final` virtual specifiers for classes and virtual functions (n2928), (n3206), (n3272)

- `override` virtual specifiers for virtual functions (n2928), (n3206), (n3272)

- Range-based `for` loops (n2930) (range-for)

- Unrestricted Unions (n2544)

- Preprocessor Condition `__has_include` (p0061r0) (p0061r1)

- Hexadecimal Floating-Point Literals (p0245r1)

- Construction Rules for `enum class` Values (p0138r2)

- Allow `typename` in template template parameter (n4051) — template template parameters are barely used (if at all) in HotSpot, but there's no reason to artificially forbid this syntactic regularization in any such uses.

## Forbidden Features

### Structured Bindings

The use of structured bindings p0217r3 is forbidden. Preferred approaches for handling functions with multiple return values include

- Return a named class/struct intended for that purpose, with named and typed members/accessors.

- Return a value along with out parameters (usually pointers, sometimes references).

- Designate a sentinel "failure" value in the normal return value type, with some out of band location for additional information. For example, this is the model typically used with `errno`, where a function returns a normal result, or -1 to indicate an error, with additional error information in `errno`.

There is a strong preference for names and explicit types, as opposed to offsets and implicit types. For example, there are folks who strongly dislike that some of the Standard Library functions return `std::pair` because `first` and `second` members don't carry any useful information.

### File System Library

The use of the File System library is forbidden. HotSpot doesn't do very much with files, and already has adequate mechanisms for its needs. Rewriting in terms of this new library doesn't provide any obviously significant benefits. Having a mix of the existing usage and uses of this new library would be confusing.

n4100 p0218r0 p0219r1 p0317r1 p0392r0 p0430r2 p0492r2 p1164r1

### Aggregate Extensions

Aggregates with base classes are forbidden. C++17 allows aggregate initialization for classes with base classes (p0017r1). HotSpot makes very little use of aggregate classes, preferring explicit constructors even for very simple classes.

### Additional Forbidden Features

- `<algorithm>`, `<iterator>`, `<numeric>`
  Not useful without standard containers or similar classes in HotSpot.

- `<bitset>` - Overlap with HotSpot `BitMap`.

- `<cassert>`, `assert.h` - HotSpot has its own `assert` macro.

- `<exception>`, `<stdexcept>` - Use of exceptions is not permitted.

- Thread support - `<thread>`, `<mutex>`, `<shared_mutex>`, `<condition_varible>`, `<future>`
  HotSpot has its own threading support.

- Streams - HotSpot doesn't use the C++ I/O library.

- `<scoped_allocator>` - Not useful without specialized allocators.

- `<string>` - Requires allocator support, similar to standard containers.

- `<typeinfo>`, `<typeindex>`
  Use of runtime type information is not permitted.

- `<valarray>` - May allocate, but is not allocator-aware.

- New string and character literals

  - New character types (n2249)
  - Unicode string literals (n2442)
  - Raw string literals (n2442)
  - Universal character name literals (n2170)

  HotSpot doesn't need any of the new character and string literal types.

- User-defined literals (n2765) — User-defined literals should not be added casually, but only through a proposal to add a specific UDL.

- Inline namespaces (n2535) — HotSpot makes very limited use of namespaces.

- `using namespace` directives. In particular, don't use `using namespace std;` to avoid needing to qualify Standard Library names.

- Propagating exceptions (n2179) — HotSpot does not permit the use of exceptions, so this feature isn't useful.

- Avoid most operator overloading, preferring named functions. When operator overloading is used, ensure the semantics conform to the normal expected behavior of the operation.

- Avoid most implicit conversion constructors and (implicit or explicit) conversion operators. Conversion to `bool` operators aren't needed because of the no implicit boolean guideline.)

- Avoid `goto` statements.

- Attributes for namespaces and enumerators (n4266 — The only applicable attribute is `[[deprecated]]`, which is forbidden.

- Variadic `using` declarations (p0195r2)

- `std::variant<>` (p0088r3) — Even if more of the C++ Standard Library is permitted, this class will remain forbidded. Invalid accesses are indicated by throwing exceptions.

- `std::any` (p0220r1) — Even if more of the C++ Standard Library is permitted, this class will remain forbidden. It may require allocation, and always uses the standard allocator. It requires RTTI.

- `std::as_const()` (p0007r1) — If sufficiently useful, HotSpot could add such a function. It would likely be added to globalDefinitions.hpp, where there are already some similar small utilities.

- `std::clamp()` (p002501) — This function is already provided in globalDefinitions.hpp.

- Parallel STL Algorithms (p0024r2) — Even if more of the C++ Standard Library is permitted, these will remain forbidden. They are built on the standard C++ threading mechanisms. HotSpot doesn't use those mechanisms, instead providing and using its own.

- Cache Line Sizes p0154r1 — HotSpot has its own mechanisms for this, using values like `DEFAULT_CACHE_LINE_SIZE`. The platform-specific implementation of the HotSpot mechanisms might use these library functions, but there is no reason to move away from the current approach. Quoting from JOSUTTIS: "... if you know better, use specific values, but using these values is better than any assumed fixed size for code supporting multiple platforms."

- `register` storage class removal p0001r1 — The `register` storage class has been removed. `register` is still a keyword, so still can't be used for normal purposes. Also, this doesn't affect the use of `register` for gcc-style extended asm code; that's a different syntactic element with a different meaning.

- Value of `__cplusplus` — Testing whether `__cplusplus` is defined or not is permitted, and indeed required. But the value should not need to be examined. The value is changed with each revision of the Standard. But we build HotSpot and (most of) the rest of the JDK with a specifically selected version of the Standard. The value of `__cplusplus` should be known and unchanging until we change the project's build configuration again. So examining the value shouldn't ever be necessary.

- Removal of `++` for `bool` (p0003r1)

- Removal of trigraphs (n4086)

## Undecided Features

This list is incomplete; it serves to explicitly call out some features that have not yet been discussed.

Some features are undecided (so implicitly forbidden) because we don't expect to use them at all. This might be reconsidered if someone finds a good use case.

Some Standard Library features are undecided (so implicitly forbidden) because, while this Style Guide forbids the use of such, they may be sufficiently useful that we want to permit them anyway. Doing so may require some idiomatic mechanism for addressing things like `assert` incompatibility, incompatibility with HotSpot's `FORBID_C_FUNCTION` mechanism, and the like.

### std::optional<>

It is undecided whether to permit the use of `std::optional<>` (p0220r1). It may be sufficiently useful that it should be permitted despite the usual prohibition against using Standard Library facilities. Use of the `value()` member function must be forbidden, as it reports an invalid access by throwing an exception.

### std::byte

It is undecided whether to permit the use of the `std::byte` type (p0298r3). It may be sufficiently useful that it should be permitted despite the usual prohibition against using Standard Library facilities.

It has been suggested that changing the HotSpot `address` type to use `std::byte` has some benefits. That is, replace

    typedef u_char*       address;
    typedef const u_char* const_address;

    using address       = std::byte*;
    using const_address = const std::byte*;

in globalDefinitions.hpp.

A specific benefit that was mentioned is that it might improve the horrible way that gdb handles our current definition of the `address` type.

    #include <cstddef>

    typedef unsigned char* address;
    typedef std::byte* address_b;

    int main() {

      char* mem;

      address addr = (address)mem;
      address_b addr_b = (address_b)mem;

      return 0;
    }

    (gdb) p addr
    $1 = (address) 0x7ffff7fe4fa0 <dl_main> "\363\017\036\372Uf\017\357\300H\211\345AWI\211\377AVAUATSH\201\354\210\002"
    (gdb) p addr_b
    $2 = (address_b) 0x7ffff7fe4fa0 <dl_main>

This needs to be explored. Some folks have said they will do so.

### String Views

It is undecided whether to permit the use of `std::string_view` (p0220r1).

HotSpot doesn't use `std::string`, but uses `char*` strings a lot. Wrapping such in a `std::string_view` to enable the use of various algorithms could be useful. But since HotSpot also doesn't permit use of `<algorithm>` and the like, that only gets the limited set of algorithms provided by the view class directly.

There is also the issue of `NUL` termination; string views are not necessarily `NUL` terminated. Moreover, if one goes to the work of making one that is `NUL` terminated, that terminator is included in the size.

There are other caveats. Permitting use of string views would require discussion of those.

### Substring and Subsequence Searching

In addition to simple substring searching, the Standard Library now includes Boyer-Moore and Boyer-Moore-Horspool searchers, in case someone wants to search really large texts. That seems an unlikely use-case for HotSpot. See p0220r1.

### `new` and `delete` with Over-Aligned Data

It is undecided whether to permit the use of dynamic allocation of overaligned types (n3396).

HotSpot currently only has a couple of over-aligned types that are dynamically allocated. These are handled manually, not going through `new` expressions, as that couldn't work before C++17.

One of the ways an over-aligned type might arise is by aligning a data member. This might be done to avoid destructive interference for concurrent accesses. But HotSpot uses a different approach, using explicit padding. Again, this is in part because `new` and `delete` of overaligned types didn't work. But we might prefer to continue this approach.

We would need to add `operator new` overloads to `CHeapObj<>` and possibly in other places in order to support this. However, it has been suggested that implementing it (efficiently) on top of NMT might be difficult. Note that `posix_memalign` / `_aligned_malloc` don't help here, because of NMT's use of malloc headers.

If we don't support it we may want to add `operator new` overloads that are deleted, to prevent attempted uses.

Alignment usage in non-HotSpot parts of the OpenJDK:

- `alignas` used once in harfbuzz, to align a variable.

- libpipewire has `#define SPA_ALIGNED` macro using gcc `aligned` attribute, but doesn't use it.

- libsleef has `#define ALIGNED` macro using gcc `aligned` attribute. It is not used for class or member declarations.

### `std::to_chars()` and `std::from_chars`

It is undecided whether to permit the use of `std::to_chars()` and `std::from_chars()` (p0067r5).

These functions provide low-level conversions between character sequences and numeric values. This seems like a good candidate for use in HotSpot, potentially replacing various clumsy or less performant alternatives. There is no memory allocation. Parsing failures are indicated via error codes rather than exceptions. Various other nice for HotSpot properties.

Note that the published C++17 Standard puts these in `<utility>`, but a defect report moved them to `<charconv>`. This also needs `<system_error>`.

This would require upgrading the minimum gcc version to 11.1 for floating point conversion support. The minimum Visual Studio version is already sufficient. The minimum clang version requirement hasn't been determined yet.

### `std::launder()`

It is undecided whether to permit the use of `std::launder()` (p0137r1).

Change to permitted if we discover a place where we need it. Or maybe we should just permit it, but hope we don't need it.

Also, C++20 revised the relevant part of Object Lifetime in a way that seems more permissive and with less need of laundering. We don't know if implementations of prior versions take advantage of the difference.

See Object Lifetime: C++17 6.8/8, C++20 6.7.3/8

### Additional Undecided Features

- Member initializers and aggregates (n3653)

- Rvalue references and move semantics

- Shorthand for nested namespaces (n4230) — HotSpot makes very little use of namespaces, so this seemingly innocuous feature probably isn't useful to us.

- Direct list initialization with `auto` (n3681) — This change fixed some issues with direct list initialization and `auto`. But we don't use that feature much, if at all. And perhaps shouldn't be using it.

- UTF-8 Character Literals (n4267) — Do we have a use-case for this?

- Fold Expressions (n4295) — Provides a simple way to apply operators to a parameter pack. HotSpot doesn't use variadic templates very much. That makes it questionable that developers should need to know about this feature. But if someone does come up with a good use-case, it's likely that the alternatives are significantly worse, because pack manipulation without this can be complicated.

- `<tuple>` — Prefer named access to class objects, rather than indexed access to anonymous heterogeneous sequences. In particular, a standard-layout class is preferred to a tuple.

- `std::invoke<>()` (n4169)

- `<chrono>` — The argument for chrono is that our existing APIs aren't serving us well. chrono provides strong type safety. We've had multiple cases of mistakes like a double seconds being treated as double milliseconds or vice versa, and other similar errors. But it would be a large effort to adopt chrono. We'd also need to decide whether to use the predefined clocks or hook up chrono to our clocks. It may be that using the predefined clocks is fine, but it's a question that needs careful study.

- `<initializer_list>` — The potential ambiguity between some forms of direct initialization and initializer list initialization, and the resolution of that ambiguity, is unfortunate.

- `<ratio>` — `<ratio>` is a compile-time rational arithmetic package. It's also fixed (though parameterized) precision. It's not a general purpose rational arithmetic facility. It appears to have started out as an implementation detail of chrono, and was extracted and promoted to a public facility in the belief that it has broader utility.

- `<system_error>` — We don't really have a generally agreed upon mechanism for managing errors. Instead, we have a plethora of bespoke ad hoc mechanisms. Managing errors is a topic of substantial discussion. `<system_error>` might end up being a part of a result from that discussion.